Sodium silicate treatment for printing papers

ABSTRACT

Integrating sodium silicate during the paper-making process, or after paper manufacture at any time prior to final image formation. Intimately associating or bonding the fiber and the silicate provides significant benefits, for example, adding additional stiffness to papers that otherwise would not be stiff enough for use in high-speed digital laser printers, and improving the quality of colored image reproduction on high speed digital laser printers. Two specific examples are transfers of financial printing on electronic digital printers to lighter weight papers, and colored image formation in advertising material where the reproductive quality of the image must be of high quality.

TECHNICAL FIELD

[0001] This invention is in the general field of printing papers usedfor image production such as papers used for wet and dry toner printers(e.g., digital high-speed laser printers and laser-type printers).

BACKGROUND

[0002] Fidelity and reproductive quality, brilliance, and gloss areimportant attributes of a great many types of printed material, such asthose used for black, single-color and multicolor printing by wet- ordry-toner digital laser and laser-type processes. Xerox DocuTech® andDocuColor®, Indigo, Heidelberg, Oce, Xeikon, Canon, and IBM and otherssell papers used for such process. Products in which these features areimportant include magazine covers and text, advertising inserts,greeting cards, postcards, calendars, direct mail advertising, financialprinting, office correspondence using word processor and office personallaser printers, etc. Photographs provide a standard against which thefinal printed materials may be evaluated. In some cases, laser printedbase stock is coated to improve its appearance.

[0003] Another consideration for print jobs with the above-describedprinters is the weight of the paper. For some applications, a lighterweight paper is desirable. Publications that are shipped or mailed-e.g.,HMO directories, health plan benefit booklets, financial prospectusesand the like-are subject to charges based on the weight of the materialmailed. Lighter weight publication mailings reduce postal chargessubstantially. Lighter weight may be desirable for making reprints orcustom prints of textbooks, training books and manuals, directories andsimilar types of publications, and for other applications. Lightweightpapers, in which the thickness normally varies with the weight, are alsodesirable for ease of handling and compactness of storage.

[0004] On the other hand, laser printers typically require a minimumstiffness in order to run smoothly. The paper must be stiff enough andhave sufficient body to pass through several points in the paper paththat can be problematic for light-weight papers. Papers having a basisweight of 50 pounds (25″×38″/500 sheets) or more are commonly used forXerox's DocuTech® printer and similar high-speed digital laser printersto avoid jams in the paper path. The inability of such printers to uselighter weight papers reduces the economics and market opportunities ofsuch printers for customized “on demand” printing for the reasonsdiscussed above.

[0005] Official correspondence may be produced by digital laser printingby a process in which the initial letter is written on a word processorand transmitted to a digital laser printer for final printing. The finalprinting for such correspondence is done on “letterhead” paperscontaining cotton fibers, the most expensive of all office papers, whichare used for the impression they convey. The market for this product isestimated at 100,000 tons. When these papers are laser printed, thefollowing phenomenon, known as ghosting, may occur. A final letter withthe desired appearance is signed, folded and placed in an envelope. Whenthe envelope is processed during mailing, particularly in postalequipment, the image printed on the letter is transferred (in a reverseimage) to the portion of the paper that contacts it as a result offolding. Thus ghosting disfigures the letter and undermines the sender'sdesired impression.

[0006] For calling cards such as those printed with a thermographicprocess, stiffness is additionally important.

SUMMARY

[0007] We have discovered that integrating sodium silicate into printingpaper provides significant benefits, for example, substantiallyimproving image quality (particularly for color images) and addingadditional stiffness to papers that otherwise would not be stiff enoughfor use in high-speed dry-toner printers.

[0008] The use of sodium silicate in printing as described in thisinvention introduces to image-producing processes and equipment, such asthose mentioned above, the ability to produce images of far greaterfidelity and reproductive quality, brilliance, and gloss than can beobtained when the same process and printer uses uncoated printing papernot treated with the invention. The printed result approachesphotographic quality.

[0009] The invention thus makes possible reproductive results withuncoated paper that are generally only possible using higher cost coatedpapers.

[0010] Essentially all applications involving black, single color andmulticolor printing with the wet and dry toner digital laser andlaser-type processes are improved by the invention's heightened colorimaging capability and print fidelity. For example, magazine covers andtext, advertising inserts, greeting cards, postcards, calendars, directmail advertising, financial printing, office correspondence using wordprocessor and office personal laser printers, etc., and as coating basestock to be coated for photographic quality laser printing.

[0011] While the paper typically is a bleached kraft printing paper, theinvention also includes other papers that comprise sodium silicateapplied as a saturation or at or near at least one paper surface, suchas bleached or unbleached sulfite paper, with or without groundwoodfiber. Thus the invention has application with newsprint and formarketing inserts.

[0012] Due to the sodium silicate treatment, the printed areas exhibithigher image fidelity and a greater degree of gloss in comparison to areference paper that has been printed in the same way, where thereference paper lacks sodium silicate and is otherwise identical incomposition and physical properties to the paper containing sodiumsilicate. The gloss in the solid and half-tone areas of the imagederives from the ink/toner and not from the surface reflectivecharacteristics of the unprinted paper. In addition, there is muchgreater fidelity of image reproduction than with an identical imageproduced in the same manner by a reference paper lacking sodium silicateand being otherwise identical in composition and physical properties tothe paper containing sodium silicate.

[0013] The resulting image more closely approaches photographic qualitythan does an identical image produced in the same manner on a referencepaper lacking sodium silicate and being otherwise identical incomposition and physical properties to the paper containing sodiumsilicate.

[0014] The invention reduces ghosting, e.g. on high quality executivecorrespondence such as correspondence laser printed on cotton-fibercontaining “letterhead” paper.

[0015] The invention is also applicable to business reply cards, a formof direct mail advertising primarily in magazines for obtaining andrenewing subscriptions and for direct advertising of product inmagazines. Papers used for this purpose must meet U.S. Postal Servicespecifications. This market is about 225,000 tons/year. Color printingand personalizing of these cards increases the return rate (order rate)over the return rate for standard black and white cards. In particular,digital laser color printers can color-print personalized business replycards. The invention includes papers that satisfy both the postalservice demands and the demands of digital laser color printers, anunusual combination of requirements.

[0016] Yet other applications for the invention are direct mailadvertising, magazine covers, and the like. Direct mail advertising is aprinted advertisement, usually for a specific product, targeted to aspecific individual, company or address. The use of color isparticularly effective for such advertising. As with the business replycards, personalization of direct mail advertising to increase theeffectiveness, and digital laser printing enables both color andpersonalization. The invention is designed for use on these printers,both because it enhances the quality of color printing and because ithas the requisite stiffness at lower cost.

[0017] Magazine covers, which have traditionally been printed on coatedpaper, can be printed on paper according to the invention, yielding highquality results comparable to coated papers, on lower cost uncoatedpaper.

[0018] The above applications are examples and are not intended to limitthe invention, which can be more generally said to be directed toprinting paper (usually bleached) comprising sodium silicate applied atleast at or near one paper surface or applied throughout the paper.Another benefit of the invention important in numerous applications isenhanced stiffness of the paper, enabling its use of lighter weightpaper on image-making machinery. Preferably, the paper is characterizedby gurly stiffness at least 5% greater than the gurly stiffness of areference paper that lacks sodium silicate and is otherwise similar incomposition and physical properties to the paper containing sodiumsilicate. (Alternatively, the reference paper should be manufacturedsimilarly to the silicate-containing paper, except that the silicate isleft out). Gurly stiffness may be measured according to standard TAPPIspecification T-543 bending resistance of paper.

[0019] The paper may be saturated with the sodium silicate solution.Preferably, however, the sodium silicate is present in a higherconcentration at or near both of the paper surfaces, to provide improvedstiffness with less weight. When applied to both sides or through thepaper, it comprises sodium silicate in an amount >about 0.03 lbs/1000square feet of paper. In another embodiment, the sodium silicate isconcentrated only at one paper surface, in which case the loading ofsodium silicate will be >about 0.015 lbs/1000 square feet. Thelocation(s) within the paper at which the NaSi may be incorporated canbe determined by controlling variables such as, for example, theviscosity of the solution from which the NaSi is applied to the paperand the wettability of the paper to the liquid vehicles, typicallywater. A film-forming compound may also be present in the paper toassist in concentrating the sodium silicate at or near one or bothsurfaces of the paper.

[0020] Typically the amount of NaSi required to obtain increased paperstiffness in accordance with the invention is greater than the amounttypically required to achieve enhanced image quality. Paper of enhancedimage quality can be obtained according to the invention by disposingthe NaSi at or near one or both surfaces of the paper.

[0021] Even with paper having a relatively low basis weight (less than50 pounds (25″×38″/500 sheets), a substantial stiffness is achieved,e.g., a gurly stiffness of ≧80 grams. We do not wish to be bound by anyspecific theory, but, based on magnification of the paper product, itappears that the sodium silicate is intimately entwined with, and evenbound to, cellulose fibers in the paper. Sodium silicate is produced bydissolving silicate in a strong application of sodium resulting in aclear solution. A strong molecular surface charge occurs and remainswith the silicate when it is finally applied to the paper in an aqueoussolution and dried. While we do not wish to be bound to any specifictheory of the invention, it appears that the sodium silicate when addedas a solution and then dried forms an intimate association with thecellulose fibers. This material then retains its electrical surfacecharge and this is highly desirable for the attraction of the ink-tonerimage forming mechanism of the laser printer. This is distinct from theaddition of fillers that do not form such an association. It alsoappears that concentration of the sodium silicate at one or both papersurfaces proves an “I-beam” effect described below.

[0022] The above described paper (as individual sheets or as a roll) canbe run through an image producing machine to produce an image on thepaper by any image producing technique including wet or dry toner laserprinting, or any other suitable image producing technique. The paper canbe produced by any conventional process that converts fiber slurry intoa bleached printing paper. The sodium silicate may be added at any pointduring the paper-production process or it can be added at any pointafter the paper production process and prior to final image productionon the paper. For example, the sodium silicate may be added by wet endaddition, by size press, by spraying or by coating on or off of apapermaking machine.

[0023] The solution of sodium silicate is added in an aqueous mixture,for example a mixture with a pH high enough to maintain sodium silicatein solution, and low enough to substantially avoid detrimental yellowingof the bleached paper (e.g., pH>6.0 and <11.0; more preferably pH 7-pH10). It will be recognized by those skilled in this art that thesolution can be adjusted to deliver the NaSi at a pH outside that range.Also preferably, a film-forming substance is included in the aqueousmixture. Generally any number of film forming substances may be used.Those in the art will recognize that there are a large number of suchsubstances available, including polyvinyl alcohol, polyvinyl acetate,hydroxyethyl cellulose, carboxymethyl cellulose, methyl cellulose,sodium alginate are film forming agents. Polyurethanes, styrene maleicanhydrides, styrene acrylic acids and styrene acrylic esters areadditional examples of film forming agents. Those in the art willunderstand that the agents are selected to be compatible with the sodiumsilicate solution, and not all of the above agents will be compatiblewith a given system.

[0024] The details of one or more embodiments of the invention are setforth in the accompanying drawings and the description below. Otherfeatures, objects, and advantages of the invention will be apparent fromthe description and drawings, and from the claims.

DETAILED DESCRIPTION

[0025] In general, any paper-making process is suitable for use with theinvention. A standard paper-making slurry containing pulp andpaper-making chemicals is contained in a headbox of the paper machine.The slurry is processed into paper. Water is removed from the slurry asit passes over a wire mesh, the paper is then pressed and dried and theresulting sheet is then treated at a size press, redried, calendared andwound into a roll. The roll may be sheeted in a separate operation.

[0026] According to the invention, sodium silicate may be introducedanywhere during or after the paper manufacture process. By way ofexample only, the silicate may be introduced at the size press.Preferably, but not necessarily, a film-forming additive is includedwith the silicate to keep the majority of the silicate at the surface ofthe paper being formed. By way of example and not limitation, two suchfilm forming substances are hydroxyethlcellulose orcarboxymethylcellusose. They are included with the silicate solutionbeing applied. A commercial hydroxyethylcellulose, Natrasol® fromHercules Inc., Wilmington Del. is a satisfactory film forming additiveat a Natrasol®:sodium silicate weight ratio of about 1 to 3. However,when we refer to increased concentration of sodium silicate at or nearthe paper surfaces, we do not mean to exclude the possibility that somesodium silicate penetrates throughout the paper. It may be desirable tosaturate the paper with sodium silicate in some cases.

[0027] The silicate used can be any of a number of commercial silicateproducts, such as sodium silicate 9 or “N” grade sodium silicate.

[0028] A. Structural Considerations With Regard to Papers Under 50Pounds

[0029] Silicate-bonded paper can be compared to a wide flanged I-beam.The silicate-bonded surfaces are the upper and lower surfaces of thebeam. The body of the sheet between the surfaces is comparable to theI-beam. The stiffness of the beam depends on the distance that separatesthe two plates and the ability to hold them apart without buckling. Thetop plate is in compression and the bottom plate is in tension whenforce is applied from the top. In use, force is applied to alternatesides of the paper, so both surfaces must have compression and tensionstrength. The body of paper between the surfaces should have thestrength to keep the plates apart.

[0030] One way to help enhance this I-beam structure is to use afilm-forming agent that keeps more of the silicate near the paper'ssurfaces, enhancing the plate strength thereby increasing stiffness. Byincreasing the relative amount of silicate that remains near the papersurface, as opposed to being absorbed by the body of the sheet, the filmformer increases stiffness.

[0031] B. Print Quality

[0032] The invention improves print quality, including improved fidelityof image, gloss, and contrast. There are many aspects to theimprovement, including improved gloss, sharper edges to half-tone dots,fewer missed dots, improved reflectance, etc. These improvementsmanifest as a perception of so-called photographic quality.

[0033] C. Rate of Addition of Silicate

[0034] Based on a size-press addition rate such that the resultingamount incorporated in the paper is at or above 0.09 pounds per MSF ofpaper, a measurable, statistically significant increase in stiffness isobserved consistently. When the rate of addition approaches or exceedsabout 0.52 pounds per MSF, a yellowing or browning may be observed asthe paper is dried. Thus, the preferred rate of addition is between 0.09pounds per MSF and 0.52 pounds per MSF.

[0035] A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.

What is claimed is:
 1. Printing paper comprising sodium silicate appliedat or near at least one paper surface, said sodium silicate optionallysaturating the paper.
 2. The printing paper of claim 1 in which thepaper is characterized by a gurly stiffness, TAPPI specification T-543bending resistance of paper, at least 5% greater than the gurlystiffness of a reference paper that lacks sodium silicate and isotherwise similar in composition and physical properties to the papercontaining sodium silicate.
 3. The printing paper of claim 1 in whichthe sodium silicate is present in a higher concentration at or near oneof the paper surfaces.
 4. The printing paper of claim 1 in which thesodium silicate is present in a higher concentration at or near bothsurfaces of the paper.
 5. The printing paper of claim 1 in which theprinting paper comprises a film-forming compound that assists inconcentrating the sodium silicate at or near one or both surfaces of thepaper.
 6. The printing paper of claim 1 and 4 in which the papercomprises sodium silicate in an amount>0.03 lbs/1000 square feet ofpaper.
 7. The printing paper of claim 3 in which the paper comprisessodium silicate in an amount>0.015 lbs/1000 square feet of paper.
 8. Theprinting paper of claim 1 in which the paper has a basis weight lessthan 50 lbs. for 500 25″×38″ sheets.
 9. The printing paper of claim 1 inwhich the paper has a basis weight greater than 50 lbs. for 500 25″×38″sheets.
 10. The printing paper of claim 1 in which the paper has a gurlystiffness of ≧80 grams.
 11. The paper of claim 1 in which the paper hasa grade stiffness less than 80 grams
 12. The printing paper of claim 1in which the sodium silicate is intimately entwined with or bound tocellulose fibers in the paper.
 13. The printing paper of claim 1 inwhich the paper contains cotton fibers.
 14. The printing paper of claim1 in which the paper meets both the U.S. Postal Service requirements forbusiness reply postal cards and the requirements of digital laser colorprinters.
 15. The printed paper of claim 1 in which the paper meetsspecifications for direct mail advertising or magazine covers.
 16. Amethod of forming an image on the paper comprising, providing a paperaccording to any of claims 1-15; and producing an image on the paper.17. The method of claim 16 in which the image is produced by a laser orlaser-like printing process.
 18. A method of producing a bleachedprinting paper comprising producing a bleached printing paper in aprocess that converts a fiber slurry into a bleached printing paper;adding sodium silicate at any point during the paper-production processor at any point after the paper production process and prior to finalimage production on the paper.
 19. The method of claim 18 in whichsodium silicate is added in an aqueous mixture.
 20. The method of claim19 in which the aqueous mixture further comprises a film-formingcompound.
 21. The method of claim 19 or claim 20 in which the aqueousmixture has a pH high enough to maintain sodium silicate in solution,and low enough to substantially avoid detrimental yellowing of thebleached paper.
 22. The method of claim 19 and 20 in which the aqueousmixture has a pH>7.0 and <11.0.
 23. The method of claim 18 in which thesodium silicate is added by wet end addition, by size press, by sprayingor by coating on or off of a paper-making machine.
 24. Printing papercomprising sodium silicate applied at least at or near one paper surfaceor saturated with sodium silicate said paper being a bleached printingpaper or unbleached sulfite printing paper, with or without groundwoodfiber.
 25. A method of laser printing on a bleached paper comprising,providing a paper comprising sodium silicate; and using a laser toproduce an image on the paper, whereby the image exhibits a superiorimage quality.
 26. A method of laser printing an image according toclaim 25, in which the only image on the paper is black.
 27. A method oflaser printing an image according to claim 25, in which the only imageon the paper is single-colored or multicolored.
 28. A method of laserprinting an image according to claims 25-27 in which solid printed areasexhibit a greater degree of gloss than do identical solid areas on areference paper which has been printed in the same way, said referencepaper lacking sodium silicate and being otherwise similar in compositionand physical properties to the paper containing sodium silicate.
 29. Amethod of laser printing an image according to claims 25-27 in which theappearance of gloss in the solid and half-tone areas of the imagederives from the ink/toner in combination with the sodium silicate andnot from the surface reflective characteristics of the unprinted paper.30. A method of laser printing an image according to claims 25-27 inwhich there is greater fidelity of reproduction than does an identicalimage produced in the same manner of a reference paper lacking sodiumsilicate and being otherwise similar in composition and physicalproperties to the paper containing sodium silicate.
 31. A method oflaser printing an image according to claims 25-27 which more closelyapproaches photographic quality than does an identical image produced inthe same manner of a reference paper lacking sodium silicate and beingotherwise identical in composition and physical properties to the papercontaining sodium silicate.
 32. The invention when produced as describedherein has clear application as a coating base for coatings developedfor digital laser printers, as the sodium silicate will further enhancethe coating's surface characteristics to react with the ink tonersthereby enhancing the image quality as well as improving stiffness.